Orthogonal frequency division multiplexing (OFDM) is an effective solution for high data rate services because of its high bandwidth efficiency and resistance to multipath fading. In order to prevent inter-carrier interference, precise frequency synchronization between the transmitter and receiver is essential and crucial; hence, CFO estimation is an important and necessary technology for OFDM systems. By transmitting known training sequences, a receiver can estimate the CFO value based on the received signals. Then, the receiver can adjust the local oscillator to compensate the carrier frequency offset.
In a coordinated multi-point (CoMP) transmission system, multiple base stations or relay stations can simultaneously transmit the same information to user equipment (UE) to improve radio coverage and overall system performance. A UE will receive and demodulate the signals transmitted from different base stations, and different received signals may have different values of CFO. Under the scenario applying CoMP transmission, a receiver must estimate multiple CFOs corresponding to the multiple received signals for compensation. Because different signals interfere with each other at a receiver, the mutual interference significantly degrades the CFO estimation performance. Hence, the conventional frequency offset estimation technology which emphasizes on single-CFO estimation is not suitable for multi-CFO estimation in CoMP-OFDM systems. So, it becomes an important issue on how to estimate multiple carrier frequency offsets simultaneously.
Technically, designing a communication method that allows a UE to be able to estimate multiple CFOs corresponding to the multiple receiving signals from different base stations in coordinated multi-point transmission systems is necessary and becomes an important issue for future fourth generation wireless communication systems.